The present invention generally relates to a multi-chip type image sensor, and more particularly to a full-size contact type image sensor which includes a plurality of integrated circuit image sensor chips. Further, the present invention is concerned with a method of producing the multi-chip type image sensor.
Recently, in the field of image scanners, there has been considerable activity in the development of a full-size contact type image sensor in order to reduce the size of an optical system. There has been proposed an image sensor which employs an integrated circuit image sensor chip using a single silicon wafer. However, there is a limit in length of such an image sensor due to the size of the used silicon wafer. Therefore, an image sensor using a single silicon wafer is not suitable for constructing a full-size image sensor. In order to form a one-dimensional image sensor capable of reading a document of an ordinary size, it is required to arrange a plurality of integrated circuit image sensor chips on the same base. Such a sensor is called a multi-chip type image sensor. A conventional multi-chip type image sensor has been disclosed in "CCD CONTACT IMAGE SENSOR", ICS87-55.
In a conventional multi-chip type image sensor as proposed, a plurality of integrated circuit image sensor chips are arranged into a line. Alternatively, a plurality of chips are arranged so that adjacent chips partially overlap with each other. Each of the integrated circuit image sensor chips is formed of silicon and has a plurality of light receiving elements which are disposed at the same pitch (interval). Each of light receiving elements is photosensitive and generates an electric signal corresponding to light irradiated onto it. The integrated circuit image sensor chips are fastened on a single alumina base by die bonding, for example.
The following is known die bonding methods. Various parameters indicative of characteristics of the following die bonding methods are shown in respective tables.
TABLE 1 ______________________________________ (Eutectic Bonding) ______________________________________ Bonding material Au--Si, Au--Ge or Au--Sn Metallization for die Unnecessary or back surface Au evaporation Working temperature 320-450.degree. C. Working atmosphere Inactive atmosphere or reducing atmosphere Ohmic property Good Heat dissipation Good Wire bonding adaptability Yes Furnace seal adaptability Yes Mass productivity Good Cost High ______________________________________
TABLE 2 ______________________________________ (Solder Bonding) ______________________________________ Bonding material Pb-based solder or Sn-based solder Metallization for die Ni, Ni--Au or Cr--Ni--Au etc. back surface Working temperature 250-350.degree. C. Working atmosphere Inactive atmosphere or reducing atmosphere Ohmic property Good Heat dissipation Good Wire bonding adaptability No only for thermocompression bonding Furnace seal adaptability Yes Mass productivity Good Cost Medium ______________________________________
TABLE 3 ______________________________________ (Resin Bonding) ______________________________________ Bonding material Ag + epoxy, Ag + polyimide, or Ag + silicon Metallization for die Unnecessary back surface Working temperature Room temp. (curing occurs at 150-200.degree. C.) Working atmosphere Air or inactive atmosphere Ohmic property Unstable Heat dissipation Poor Wire bonding adaptability Yes Furnace seal adaptability Yes only for polyimide system Mass productivity Excellent Cost Low ______________________________________
TABLE 4 ______________________________________ (Glass Bonding) ______________________________________ Bonding material PbO--B.sub.2 O.sub.3 -based glass Metallization for die Unnecessary back surface Working temperature 420-600.degree. C. Working atmosphere Air Ohmic property Poor Heat dissipation Poor Wire bonding adaptability Yes Furnace seal adaptability Yes ______________________________________
The above-indicated die bonding methods require high working temperatures. In the present state of the field, the resin bonding method is better than the other bonding methods in view of reliability, work efficiency and mass productivity.
In the resin bonding method, a heat curing type adhesive for die bonding is coated on a base to a thickness of about tens of microns with a width corresponding to that of sensor chip at room temperature by a dispense process, a stamp process or a screen printing process. Then, a plurality of sensor chips are located on the coated adhesive and pressed against the base. Thereafter, the sensor chips are subjected to a batch process by use of an oven in which the sensor chips are subjected to a heat curing process for one to two hours at a heating temperature between 120 and 250.degree. C.
The following conductive adhesive agents for use in die bonding are available in a market. Various parameters indicative of properties of the following conductive adhesive agents are shown in respective tables.
TABLE 5 ______________________________________ (Chemitight CT212 manufactured by Toshiba Chemical Corp) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 200.degree. C./1 hr Volume resistivity (.OMEGA. .multidot. cm) 0.6 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) 6 .times. 10.sup.-3 Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 5.vertline.5 ______________________________________
TABLE 6 ______________________________________ (EN-4000 manufactured by Hitachi Chemical Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 175.degree. C./1 hr Volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) 0.6 .times. 10.sup.-3 Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.10 ______________________________________
TABLE 7 ______________________________________ (EN-4070X-13 manufactured by Hitachi Chemical Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 150.degree. C./1 hr to 250.degree. C./40 sec Volume resistivity (.OMEGA. .multidot. cm) 3.3 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.10 ______________________________________
TABLE 8 ______________________________________ (CRM-1038 manufactured by Sumitomo Bakelite Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 200.degree. C./1 hr .about. 170.degree. C. /20 sec + 350.degree. C. /20 sec Volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) 3 .times. 10.sup.-3 Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.10 ______________________________________
TABLE 9 ______________________________________ (CRM-1058 manufactured by Sumitomo Bakelite Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/polyimide Curing condition (temp./time) 150.degree. C./1 hr .about. 250.degree. C./1 hr Volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.20 ______________________________________
TABLE 10 ______________________________________ (Ablebond 84-1 LMI manufactured by Ablestic Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 150.degree. C./1 hr volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) 4.5 .times. 10.sup.-3 Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.10 ______________________________________
TABLE 11 ______________________________________ (Ablebond 71-1 LMI manufactured by Ablestic Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/polyimide Curing condition (temp./time) 150.degree. C./30 min .about. 275.degree. C./30 min volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.5 ______________________________________
TABLE 12 ______________________________________ (EPO-TEK H-20ELC manufactured by Epoxy Technology Corp.) ______________________________________ State (Compounding ratio) two-liquid type (1:1) Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 120.degree. C./15 min volume resistivity (.OMEGA. .multidot. cm) 3 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) 4 .times. 10.sup.-3 Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 30.vertline.-- ______________________________________
TABLE 13 ______________________________________ (EPO-TEK H35-175M manufactured by Epoxy Technology Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 180.degree. C./1 hr volume resistivity (.OMEGA. .multidot. cm) 2 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.10 ______________________________________
TABLE 14 ______________________________________ (Du Pont 4621D manufactured by Du Pont) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 175.degree. C./1 hr volume resistivity (.OMEGA. .multidot. cm) 4 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 20.vertline.10 ______________________________________
TABLE 15 ______________________________________ (C-990 manufactured by Amicon Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent none Composition (filler/resin) Ag/epoxy Curing condition (temp./time) 155.degree. C./1 hr volume resistivity (.OMEGA. .multidot. cm) 6.5 .times. 10.sup.-4 Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.5 ______________________________________
TABLE 16 ______________________________________ (C-940 AXLC manufactured by Amicon Corp.) ______________________________________ State (Compounding ratio) one-liquid type Solvent available Composition (filler/resin) Ag/polyimide Curing condition (temp./time) 175.degree. C./30 min.about. 275.degree. C./30 min volume resistivity (.OMEGA. .multidot. cm) -- Heat conductivity (cal/cm .multidot. sec. .degree.C.) -- Extracted impurity (Cl.sup.- .vertline.Na.sup.+) 10.vertline.20 ______________________________________
In multi-chip type image sensors, integrated circuit image sensor chips must be held on the base so that the distance between adjacent light receiving elements provided in adjacent integrated circuit image sensor chips is identical to the distance between adjacent light receiving elements within the same sensor chip. Unless the distance satisfies the above-mentioned condition, the continuity of arrangement of light receiving elements is disturbed and therefore the reading quality is degraded.
As described in the aforementioned paper, the chip mounting methods by use of die bonding present a disadvantage that a positional deviation or error (misregistration) of sensor chips occurs during heat curing (die bonding). The positional deviation of sensor chips causes an error in the pixel pitch between neighboring light receiving elements of adjacent sensor chips (pixel pitch). The caused error deteriorates the reading quality.
The positional error of sensor chips caused during die bonding is further described. The thermal expansion coefficient of a sensor chip formed of silicon, .alpha..sub.Si is approximately 3.5.times.10.sup.-6 [1/.degree.C.], and the thermal expansion coefficient of a base formed of alumina .alpha..sub.B =.alpha.(Al.sub.2 O.sub.3) is nearly equal to 6.5.times.10.sup.-6 [1/.degree.C.]. Curing temperatures of the aforementioned adhesives for use in die bonding are higher than 100.degree. C. When sensor chips are fixed on a base by the heat curing process, the sensor chips and the base expand. However, it is noted that the degree of expansion for the silicon sensor chips is different from (less than) the degree of expansion of the alumina base. After heating the assembled of the sensor chips and the base to the curing temperature, temperature is decreased to room temperature. During this process, sensor chips contracts and therefore the size of sensor chips at room temperature becomes smaller than that of those before heat curing. As a result, the pixel distance between adjacent sensor chips becomes wider than that obtained before heat curing. In other words, the distance of adjacent elements within the same sensor chip becomes smaller than that obtained before heat curing. Particularly, sensor chips used for the full-size contact type image sensor are long and narrow. Therefore, the misregistration between each sensor chip and the base resulting from a difference in thermal expansion coefficient occurs greatly. For a full-size contact type color image sensor, a deviation of the tone of color occurs at joint portions of adjacent sensor chips. This causes a great deterioration in image quality.
The aforementioned paper teaches that a conventional image sensor has a misregistration equal to .+-.15 .mu.m. Such a misregistration cannot satisfy specifications required for high-density image sensors, e.g., 400 dpi (dots per inch) for A3 size.